Process of treating mixtures of alkali stannates, arsenates, and antimonates



Patented Feb. 18, 1930' UNITED STATES PATENT OFFICE ARTHUR E. HALL, OF PERTH AMBOY, NEW JERSEY, ASSIGNOR TO AMERICAN SMELT- ING AND REFINING COMPANY, OF NEW YORK, N. Y-, A CORPORATION OF NEW JERSEY PROCESS OF TREATING MIXTURES OF ALKALI STANNATES, ARSENATES, AND

ANTIMONATES No Drawing.

This invention relates to a process for treating mixtures of alkali stannates, arsenates and antimonates, and, more particularly, to a process for treating fused mixtures of 5 the above substances, such as are obtained as a by-product in the refining of metals.

The invention further relates to a process of treating alkali stannates, arsenates and antimonates in combination with caustic alkali and alkali chloride, for the recovery of the tin, arsenic and antimony.

The invention still further relates to a process for treating the above combination of substances for the recovery of the tin, arsenic and antimony either as metals or as salts of a marketable state of purity, and for the recovery of the free caustic alkali and alkali chloride.

The invention further consists in the new and novel features of operation and the new and original arrangements and combinations of steps in the process hereinafter described and more particularly set forth in the claims.

In certain processes for the refining of metals, such as, for example, the refining of lead by the addition of caustic, a by-product or dross is formed consisting of oxy-salts,of arsenic, antimony and tin, which are so closely combined as to be capable of separation only with great difliculty. The dross in addition to containing the above elements, contains an excess of caustic with which the metal was previously treated. It is therefore desirable to separate and recover not the caustic or other drossing reagent in order that the same may be re-utilized in further metal refining processes.

In accordance with the present invention, the various above mentioned substances are separated by utilizing the various properties of the substances by means of which they are capable of selectively combining or precipitating. For example, the oXy-salts of antimony and tin are extremely insoluble in hot alkali hydroxide solutions, while the oxysalts of arsenic are largely soluble therein. Furthermore, the oxy-salts of arsenic are insoluble in the same solution when cold. In accordance with the present process, thereonly the arsenic, antimony and tin, but also Application filed March 5, 1927. Serial No. 173,220.

fore,'the salts of arsenic are dissolved from the combination of arsenic, antimony and tin oxy-salts by means of a hot caustic solution, and the solution after being removed from the insoluble residue is cooled to cause the arsenic salts to crystallize.

The present process further takes advantage of the difference in solubility of the stannates and antimonates in hot water, whereby the salts of the stannates may be obtained While leaving the antimonates apparently undissolved. The stannate solution may then be electrolyzed for the recovery of the tin, being previously treated, if desired, to remove various other impurities.

More particularly, the process may be carried out in a plurality of steps, such as by digesting the mixture of alkali stannates, arsenates and antimonates in a hot solution of alkali hydroxide and alkali chloride, or other equivalent material in which the alkali antimonates and stannat-es are insoluble, or only slightly soluble and the alkali arsenates are substantially all soluble.

The solution may then be separated from the undissolved alkali antimonates and stannates, preferably while still hot, by filtration, decantation, or other suitable means.

The hot solution of alkali hydroxide, alkali chloride, and alkali arsenates, contained in the filtrate from the preceding step may be cooled to crystallize out the alkali arsenates,

and the crystallized alkali arsenates, sep-.

arated by filtration, dccantation, or otherwise, from the residual caustic mother llquor which may then be evaporated for the recovcry of the alkali hydroxide and -alkali chloride contained therein.

The alkali antimonates and stannates which were obtained as a filter cake or sludge may be digested in hot Water to dissolve the alkali stannates and leave alkali antimonates undissolved. The latter may be separated from the alkali stannate solution, by filtration, decantation, or other suitable means.

The alkali stannate solution may be treated with metallic tin, stannous salts, sodium sulphide, or other suitable reagent to precipitate lead, copper, and other impurities that would interfere with subsequent operawhich may be followed in carrying out this invention, the treatment of a fused mixture of. sodium antimonate, arsenate, and stannate, together with free sodium hydroxide and sodium chloride, resulting from the refining of lead by the Harris process, will be described. The composition of such a mixture will vary widely, depending upon the analysis of the lead bullion from the refining of which it results, but the following analysis is typical:

Per cent Arsenic 2. 7 Antimony 13. 3 Tin 5.0 Free caustic soda 42.2 Sodium chloride 12. 5

In the treatment of such a mixture according to this invention the fused mixture may be poured in a thin stream into a much larger circulating stream of water or of a solution of caustic soda and sodium chloride more dilute than the solution which it is desired to have for the final digestion of the mixture. The fused mixture is thus simultane ously cooled and granulated into small particles while the heat given up by it is utilized in heating. the water or solution. If desired, however, the fused mixture may be first cooled and pulverized and later added to the water or dilute caustic soda and sodium chloride solution.

A sufficient amount of the mixture of sodium antimonate, arsenate, and stannate, sodium chloride, and free caustic soda should be added to the water or solution to raise the sodium chloride and free caustic soda content of the latter to such a concentration that the sodium antimonate and sodium stannate will be substantially insoluble and the sodium arsenate substantially all soluble. It has been found that this condition holds true in a solution saturated or nearly saturated with sodium chloride and containing enough free caustic soda to raise its specific gravity to approximately 1.4 as compared to pure water.

After the necessary amount of the .fused mixture has been added to the water or solution, the whole is thoroughly digested while hot, say at a temperature of 90 C. or over, in a suitable agitating apparatus such as a steel tank provided with a mechanically driven agitator, until the sodium arsenate is dissolved.

It is important that in this digestion the solution should have approximately the con centration in caustic soda and sodium chloride specified above, since in somewhat more dilute solutions the sodium stannate is soluble to a marked degree.

After the completion of the digestion as described the insoluble sodium antimonate and sodium stannate may be separated from the solution by filtration on a suitable filter such as a vacuum filter or a filter press, but if desired the separation may be accomplished by decantation or otherwise. The so dium antimonate and sodium stannate may be washed with a solution of sodium chloride and free caustic soda of 1.4 sp. gr., containing no sodium arsenate, in order to wash the latter completely out of the antimonate and stannate. Water alone may not be used for this washing as the sodium stannate would be soluble therein.

The filtrate obtained from the above filtration consists principally of sodium arsenate, free caustic soda, and sodium chloride. It may be pumped to suitable cooling apparatus such as tanks equipped with cooling jackets or coils and with agitators, Where it may be cooled toapproximately atmospheric temperature, at which temperature the sodium arsenate is largely insoluble and crystallizes out. After cooling, the crystals are separated from the solution by filtration in a suitable filter, such as a plate and frame press or a sweetland filter, or by centrifuging. The crystals of sodium arsenate may be sold as such or may be used for the production of insecticides such as calcium arsenate.

The filtrate after separation of the sodium arsenate is evaporated in cast iron kettles or other evaporators for the removal of water and the production of a fused mixture of caustic soda and sodium chloride, which can .be reused for refining a further quantity of lead bullion.

The sodium antimonate and sodium stannate remaining undissolved after the digestion in the solution of caustic soda and sodium chloride of a specific gravity of approximately 1.4 as previously described. are, after separation from the solution by filtration or otherwise, thoroughly agitated with hot water in a suitable apparatus such as an iron tank equipped with steam coils for heating and with a motor driven agitator. An amount of water is preferably used such as will give a solution containing approximately 45 grams of tin per liter, but this may vary within rather wide limits according to the strength of solution desired. The agitation and heating should be continued until substantially all the sodium stannate and other water-soluble salts are dissolved, leaving the sodium antimonate undissolved.

The resulting pulp is then pumped to a suitable filter, such as an ordinary plate and frame filter press, and the solution is separated from the insoluble sodium antimonate; or the separation may be effected by decantation or other means if preferred. The sodium antimonate is washed with suflicient water to wash out the dissolved salts; the wash water may later be used for the lixiviation of a succeeding batch of sodium antimonate and stannate. The washed sodium antimonate is dried and smelted with the addition of carbon for reduction to marketable metallic antimony, or if sufliciently pure it may be marketed as sodium antimonate.

The filtrate from the filter press consists of a solution of sodium stannate containing also some sodiumchloride and sodium hydroxide and minor amounts of various impurities. It may also contain a certain amount of sodium arsenate if the latter has not been thoroughly washed out of the mixed sodium antimonate and stannate after the original leach in strong caustic liquor, but the amount of sodium arsenate will be small compared to the amount contained in the original mixture of antimonate, arsenate, and stannate before treatment.

The above filtrate containing the sodium stannate is collected in a second tank and agitated with powdered tin or other form of metallic tin, stannous salts, sodium sulphide, or other suitable reagent, to precipitate lead, copper, antimony, tellurium, or other deleterious impurities. The precipitated impurities are thenseparated from the solution by filtration or other suitable'means.

If desired the metallic tin or other reagent for the purification of the sodium stannate solution may be added before the separation of the solution from the sodium antimonate, in which case the sodium antimonate and the precipitated impurities are filtered oif together; this procedure is somewhat simpler than performing the two steps separately, but does not yield so pure a quality of sodium antimonate.

After the purification of the solution as above described it is pumped to electrolyzing vats, where the tinis deposited under the usual conditions for the electrodeposition of tin'from alkali stannate solution, i. e., the temperature is maintained at a temperature of approximately 90 0., a coating of molten parafiine is maintained on the surface of the electrolyte to prevent undue evaporation and formation of spray, insoluble iron anodes and tin cathode starting sheets are used, and the current density is maintained at approximately 10 to 20 amperes per square foot. Simultaneously with the deposition of the tin the sodium combined as sodium stannate is released by the electrolysis and forms caustic soda, thus increasing the free caustic. content of the solution.

After the tin content of the solution has been reduced to the desired point by electrodeposition, the solution is pumped to a suitable evaporator, such as a cast iron kettle heated from below, for the removal of water and the production of fused caustic soda for further use.

If desired, the spent electrolyte from the electro-deposition of the tin may, before being evaporated, be cooled to approximately atmospheric temperature, at which temperature sodium arsenate is substantially insoluble, in order to crystallize out any sodium arsenate that may be contained therein. If the prior operations described have been carefully performed however, the amount of sodium arsenate present at this stage will be very small.

In accordance with the above described process the oxy-salts of arsenic, antimony and tin, which are present in the dross obtained in the refining of lead by the so-called Harris process, may be treated for the separation and recovery of the various elements contained therein, and also for the recovery and re-utilization of the caustic and alkali chloride.

The process provides for a cheap and efiicient means for effecting the above separation without the employment of expensive apparatus, which in View of the comparatively small value of the metals themselves with respect to the quantity of material handled, is an essential consideration. Heretofore it has been considered extremely difiicult to separate the arsenic, antimony and tin when fused in an intimately combined state such as exists with the dross from metal refining processes.

Obviously, the invention is applicable to the separation of the above substances from other combinations than the above mentioned dross. It would, in fact, be especially useful for the separation of arsenic and tin when uncombined with antimony, irrespective ofthe desirability of, recovering the caustic from the resultant filtrates.

Although certain novel features of the invention have been shown and described and are pointed out in the annexed claims, it will be understood that various omissions, substitutions and changes in the several steps of the process and in its operation may be made by those skilled in the art without departing from the spirit of the invention.

What I claim is:

1. The process for separating alkali stannates, arsenates and antimonates which comprises dissolving the alkali arsenates in an alkaline solution of such concentration that the stannates and antimonat'es are comparatively insoluble therein and separating the solution from the undissolved alkali antimonates and stannates.

1 2. The process for separating arsenates from mixtures of stannates, arsenates and antimonates, which comprises dissolving said arsenates in a hot caustic solution of such concentration that the stannates and antimonates are comparatively insoluble therein, and cooling the hot caustic solution to crystallize ou the alkali arsenates.

3. The process for separating arsenates from mixtures of alkali stannates, arsenates and antimonates, which comprises dissolving said arsenates in a hot caustic solution of alkali chloride of such concentration that the alkali stannates and antimonates are comparatively insoluble therein, separating the solution from the undissolved alkali antimonates and stannates, cooling the hot caustic solution of alkali chloride and alkali arsenates to crystallize out the alkali arsenates, separating the crystallized arsenates from the residual caustic mother liquor and recovering the alkali chloride from said caustic mother liquor by evaporation.

4. The process for separating alkali stannates, arsenates and antimonates which comprises dissolving said arsenates in a hot caustic solution of such concentration that the alkali stannates and antimonates are comparatively insoluble therein, separating the solution from the undissolved alkali antimonates and stannates and dissolving the alkali stannates in a hot neutral solution whereby the alkali antimonates remain undissolved.

5. The process for separating alkali stannates, arsenates and antimonates which comprises dissolving said arsenates in a hot caustic solution of alkali chloride of such concentration that the alkali stannates and antimonates are substantially insoluble therein, separating the solution from the undissolved alkali antimonates and stannates, dissolving the alkali stannates in hot water and separating the alkali stannate solution from the undis solved alkali antimonates.

6. The process for separating alkali stannates, arsenates and antimonates which comprises dissolving said arsenates in a hot caustic solution of alkali chloride of such concentration that the alkali stannates and antimonates are substantially insoluble therein, separating the solution from the undissolved alkali. antimonates and stannates, dissolving the alkali stannates in hot water and separating the alkali stannate solution from the undissolved alkali antimonates and treating the alkali 'stannate solution with a suitable reagent to precipitate lead, copper, antimony, tellurium, and other deleterious impurities.

7 The process for separating antimonates from mixtures of alkali stannates, arsenates and antimonates which comprises dissolving said arsenates ina hot caustic solution of alkali chloride of such concentration that the alkali stannates and antimonates are insoluble therein and dissolving the stannates in hot water whereby the alkali antimonates remain undissolved.

8. The process of treating mixtures of oxysalts of arsenic, tin and antimony, which comprises dissolving said arsenic salts in a hot caustic chloride solution of such concentration evaporating the filtrate for the recovery of all fused caustic.

In testimony whereof I have hereunto set my hand.

ARTHUR E. HALL. 

